A Born-Haber cycle is a thermochemical cycle that includes all the enthalpy changes involved in the formation of an iconic compund. We construct it by starting with the elements in their standard states. All elements in their standard states have zero enthalpy by definition

The lattice enthalpy of an iconic solid is the enthaply change when one mole of the solid in its standard state is formed from its icons in the gaseous state. For example:

The problem is that you cannot react gaseous sodium icons with gaseous chloride ions and measure the enthalpy change as solid sodium chloride is formed. The value for the lattice enthaply has to be determined indirectly. We make use of changes for which data are available and link them together in an enthaply cycle. This enthalpy cycle is based on the formation of the compound from its elements in their standard states. For sodium chloride this reaction is:

You can imagine this reaction occurring in a number of steps, one of which corresponds to the lattice enthalpy:

1. The elements in their standard states are turned into gaseous atoms
2. The gaseous atoms become gaseous ions
3. The gaseous ions come together to form a solid sodium chloride

The resulting enthalpy cycle is known as the Born-Haber cycle. The below figure shows the Born-Harber cycle for sodium chloride

is the standard enthalpy change of formation for sodium chloride;

  is made up of two components:

i) the standard enthalpy change of atomisation of sodium:

ii) the standard enthalpy change of atomisation of chlorine:

The standard enthalpy change of atomisation of an element is the enthalpy change that occurs when one mole of atoms in the gaseous state is formed form the element in its standard state. Note that it is always quoted per mole of the atom formed.

is made up of two components.

i. the first ionisation enthalpy of sodium, that is the enthalpy change that occurs when one mole of sodium atoms in the gaseous state each lose the single outer electron to form sodium ions

ii. the first electron affinity of chlorine, that is the enthalpy change that occurs when ine mole of chloring atoms in the gaseous state each gain a single electron to form chloride ions:

is the lattice enthaply for sodium chloride

Applying Hess's law to this Born-Haber cycle gives:

So, in conclusion Born- Haber cycles are often drawn as enthalpy level diagrams in which the various enthalpy changes are drawn to scale. These can be helpful as they provide a visual display of how each enthalpy cycle contributes to the cycle. This in turn helpw when looking for explanations for  trends in properties.